1. Technical Field
The present invention relates to a photoelectric conversion apparatus and, more particularly, to a photoelectric conversion apparatus utilizing an avalanche effect for amplifying optically generated carriers by impact ionization.
In particular, the present invention relates to a low-noise photoelectric conversion apparatus suitably used in a photometric sensor for a camera, or an image sensor for an image reading apparatus such as a facsimile system, a copying machine, or the like, or a light-receiving sensor in an optical communication apparatus, or the like.
2. Related Background Art
In information transmission techniques in a video information system, optical communication, and other industrial and commercial fields, which utilize light as a medium of an information signal, a semiconductor light-receiving element for converting a light signal into an electrical signal is one of the most important and basic constituting elements, and a large number of elements have already been commercially available. In general, semiconductor light-receiving elements are required to have a high signal-to-noise ratio against its photoelectric conversion characteristics.
Of these elements, an avalanche photodiode utilizing an avalanche effect (to be abbreviated to as an APD hereinafter) is a promising candidate of a semiconductor light-receiving element satisfying such requirements since it has a high gain and a high response speed.
A large number of APDs have already been commercially available as, in particular, semiconductor light-receiving elements in optical communication systems which elements use compound semiconductors such as InGaAs, and the like as materials. Furthermore, developments have been made to improve basic performance, e.g., low noise, high-speed response, a high gain, and the like of elements. In addition, demand has arisen for an application to other fields, for example, visible light light-receiving elements.
FIG. 1 is a longitudinal sectional view showing a structure of a conventional APD for optical communication.
In FIG. 1, reference numeral 101 denotes an n.sup.+ -type InP layer; 102, an n-type InGaAs layer; 103, an n-type InP layer; and 104, a p.sup.+ -type InP layer. Note that the n-type InGaAs layer 102, the n-type InP layer 103, and the p.sup.+ -type InP layer 104 are formed to have a mesa pattern. A p-type electrode 106 is formed on the upper surface of the p.sup.+ -type InP layer 104 to leave a window 105, and an n-type electrode 107 is formed on the lower surface of the n.sup.+ -type InP layer 101. Reference numeral 108 denotes a passivation film. When the p- and n-type electrodes 106 and 107 are biased in opposing directions, and light is radiated from the window 105, light is absorbed by the n-type InGaAs layer 102 (to serve as a light absorption layer), thus performing photoelectric conversion. More specifically, electron-hole pairs formed in the n-type InGaAs layer 102 respectively migrate toward the n- and p-type electrodes 107 and 106. Since the n-type InP layer 103 (serving as a multiplication layer) has a strong electric field, it causes an avalanche phenomenon for forming a large number of electron-hole pairs during a hole migration process, thus causing a multiplication effect for forming a plurality of electron-hole pairs per photon. As a result, very weak incident light can be detected. However, the conventional structure suffers from two drawbacks. That is, a practical multiplication effect is as small as about twice, and over-multiplication noise is generated due to a fluctuation present in a multiplication process, thus decreasing a signal-to-noise ratio.
It is known according to an article by R. J. McIntyre (IEEE Transactions on Electron Devices, Ver. 13, pp. 164-168 (January, 1966)) that noise generated during an avalanche multiplication process strongly depends on a ratio k=.beta./.alpha. of an electron ionization ratio a to a hole ionization ratio .beta..
Note that the electron ionization ratio is a ratio of generating electron-hole pairs by impact ionization when electrons are accelerated by an electric field. The hole ionization ratio is a ratio of impact ionization by holes. Furthermore, this article also reveals that in order to obtain a low-noise APD, k is decreased when electron multiplication is performed, while k is increased when hole multiplication is performed. More specifically, it is important for obtaining a high signal-to-noise ratio in an APD to avalanche-multiply only carriers having a larger ionization ratio in materials having considerably different carrier (electron or hole) ionization ratios. This article also describes that an over-noise index F becomes 2 as a low-noise limit attained when only one carrier is avalanche-multiplied. In an ideal state wherein no noise is generated at all, F should become 1. The article also implies that a certain mechanism for generating noise is present at the limit of F=2. As this mechanism, the phenomenon occurs. In this phenomenon, locations, where ionization (inverse Auger effect) as a fundamental process of avalanche multiplication occurs when avalanche multiplication is performed, fluctuate independently, and such fluctuations are multiplied, thus causing a fluctuation in multiplication ratio.
In consideration of the above-mentioned findings, in order to perform avalanche multiplication free from noise generation, 1 a location where ionization as its fundamental process must be specified in an element, and 2 a probability of ionization at the ionization location must be specified. Furthermore, in order to perform high-gain avalanche multiplication, the probability of ionization must be approximate to 1 as much as possible.
In consideration of the above-mentioned two drawbacks, i.e., a small degree of multiplication, and a decrease in signal-to-noise ratio (SN ratio), as an APD for optical communication, for example, F. Capasso et. al. proposed a low-noise APD which is prepared using a compound semiconductor mainly belonging to Group III-V using molecular beam epitaxy (MBE) and can be used in an optical communication system in Japanese Laid-Open Patent Application No. 58-157179, and IEEE Electron Device Letters, Ver. EDL3 (1982), pp. 71-73.
The characteristic feature of this element is a multilayered heterojunction structure. In this structure, a large number of semiconductor layers in which a band gap is continuously changed from a narrow side to a wide side by changing composition ratios of its constituting materials (for example, if compound semiconductors belonging to Group III-V are constituting materials, a composition ratio of Group III semiconductors to Group V semiconductors) are stacked, and ionization is promoted by utilizing stepwise transition portions of energy bands (to be referred to as a stepback structure hereinafter). The schematic structure of the proposed element will be described below with reference to FIGS. 2A to 2C.
FIG. 2A is a sectional view of this element. Stepback structure layers 201, 203, 205, 207, and consisting of five layers serving as multiplication layers are sandwiched between p- and n-type semiconductor layers 211 and 215 which serve as light absorption layers. An electrode 213 is in ohmic-contact with the p-type semiconductor layer 211, and an electrode 214 is in ohmic-contact with the n-type semiconductor layer 215.
FIG. 2B is a view showing a structure of energy bands of inclined band-gap layers when no bias is applied to this element, and illustrates three inclined band-gap layers. These layers have compositions for linearly changing a band gap from a narrow band-gap Eg2 to a wide band-gap Eg3.
Magnitudes of stepbacks of conduction and valence bands are represented by .DELTA.Ec and .DELTA.Ev. As will be described later, in order to mainly facilitate ionization of electrons, .DELTA.Ec is set to be larger than .DELTA.Ev.
FIG. 2C is a view showing a structure of energy bands when a reverse bias voltage is applied to this element. Note that the reverse bias voltage need not be a stronger electric field than that for an APD shown in FIG. 1.
When light is incident from the p-type semiconductor layer 211, light absorbed by the p-type semiconductor layer and the respective stepback structure layers is subjected to photoelectric conversion like in the above-mentioned APD, and formed electron-hole pairs migrate toward n- and p-type semiconductor layers 215 and 211, respectively. However, a difference from the APD shown in FIG. 1 is that when the energy step .DELTA.Ec of each stepback structure (for electrons, .DELTA.Ev for holes) becomes larger than an ionization energy, electrons are ionized to generate electron-hole pairs, and cause a multiplication effect. Of course, since the respective stepback structure layers perform the same operation, the multiplication effect occurs 2.sup.n times with respect to the number of layers n. For example, ideally, since a hole ionization ratio can be suppressed to be very smaller than an electron ionization ratio when .DELTA.Ec&gt;&gt;.DELTA.Ev=0 is set, low noise can be realized as compared to the above-mentioned APD.
More specifically, a bias voltage is applied to generate an electric field (drift electric field) so as to convert at least the inclined band-gap layers 201, 203, 205, and 209 of the stepback structure layers into depletion layers, and to cause a carrier drift but not to cause ionization in the inclined band-gap layers. Light h.upsilon. is absorbed by a depletion region next to the p-type semiconductor layer 211, i.e., by the inclined band-gap layer 203, and generates electrons in the conduction band and holes in the valence band, respectively. The generated electrons drift in the layer 203 toward a stepback of the first conduction band. Since the energy step .DELTA.Ec has already been present at the stepback, and the electrons can compensate for an energy necessary for causing ionization by this energy step, a probability of causing ionization by the electrons immediately after the stepback is increased. When .DELTA.Ec is equal to or larger than the ionization energy of the electrons, or if a short energy can be supplied from the drift electric field even when it is smaller than the ionization energy of the electrons, the probability of causing ionization after the stepback can be sufficiently approximate to 1. When ionization occurs, one electron becomes two electrons and one hole. The two electrons drift in the inclined band-gap layer 203 toward the second stepback, and cause the same phenomenon as described above at the second stepback. Meanwhile, the hole generated in a front portion in the inclined band-gap layer 203 drifts forward, i.e., in a direction opposite to the electrons, and reaches the first stepback. If the energy step .DELTA.Ev which does not cause ionization of the hole has already been present in the valence band of the first stepback, the drifting hole is kept migrating forward. If there is a positive energy step in front of the hole shown in FIG. 2C, the hole is scattered or stored at the stepback, but does not cause ionization. In this manner, a drift and ionization of electrons and a drift of holes are repeated at the respective inclined band-gap layers and stepbacks, thus multiplying the number of carriers. Finally, electrons multiplied by ionization reach the n-type semiconductor layer, and are extracted as an electron current from a layer which is in ohmic-contact with the n-type semiconductor layer, while holes reach the p-type semiconductor layer and are extracted as a hole current from a layer which is in ohmic-contact with the p-type semiconductor layer.
As can be understood from the above description, the multilayered heterojunction structure in which a large number of semiconductor layers in which a band gap is continuously changed from a narrow side to a wide side by changing a composition ratio of its constituting materials are stacked, and which utilizes stepbacks formed at that time to promote ionization can constitute a low-noise APD which embodies an idea in that a location of ionization is specified, and a probability of ionization is approximate to 1 as much as possible, as described above.
Although the above-mentioned element structure is one means for realizing a low-noise APD, various limitations are exerted in practice upon formation of an element having such a structure.
In order to obtain an element having stepback structure layers which can promote ionization by only changing a composition ratio of the constituting materials, constituting materials and a formation method are limited. For example, materials which can constitute an element having such a structure include Group III-V compound semiconductors, e.g., one obtained by growing AlGaAsSb/GaSb on a GaSb substrate, one obtained by growing InGaAlAs/InGaAs on an InP substrate, and one obtained by growing InGaAsSb/GaSb on a GaSb substrate, or a material obtained by growing HgCdTe as a Group II-VI compound semiconductor on a lattice-matched substrate.
However, Ga, As, Hg, Cd, and the like used herein are materials posing a lot of problems in industrial use since they have high toxicity, and are rare and expensive elements.
These compounds are prepared by molecular beam epitaxy (MBE). However, the MBE requires an ultra high vacuum, has a low growth rate of a semiconductor, and is not suitable for a large-area structure, thus disturbing mass-production. Furthermore, in the MBE, a growth temperature of a semiconductor is typically as high as 500 to 650.degree. C. Therefore, when such a light-receiving element is stacked on a semiconductor device on which integrated circuits, and the like have already been formed, the existing semiconductor device may be damaged.
In order to form such a low-noise APD, the composition ratio of these materials must be changed to always cause ionization at stepbacks. For this purpose, the composition ratio of materials must be determined in consideration of a lattice matching property which does not cause a trap level of a heterojunction interface, and an electron affinity having a stepback energy step equal to or larger than an ionization energy. As a result, the band gap of an actually formed APD is limited.
For example, when the first materials were used, it was experimentally confirmed that in a lattice-matched structure, a band gap of a material (GaSb) having a narrowest band gap was 0.73 eV, a band gap of a material (Al.sub.1.0 Ga.sub.0.0 As.sub.0.08 Sb.sub.0.92) having a widest band gap was 1.58 eV, a maximum band gap difference was 0.72 eV on the side of the conduction band and 0.13 eV on the side of the valence band, and an electron ionization energy was 0.80 eV (GaSb). 0.08 eV as a shortage of the electron ionization energy at the stepback is supplied from an electron drift electric field. In this element, a leaking current (dark current) signal which is generated when no light is radiated tends to be generated, and increases noise components. As a result, a low-noise element cannot be obtained, thus posing a serious problem. Causes for generating a dark current include carriers injected from an ohmic-contact layer (electrode outside the element), carriers thermally generated via a defect level in the element or a hetero interface level, and the like. In this element, an effect for blocking injected carriers is consequently obtained by arranging the p- and n-type semiconductor layers. However, any conscious and sufficient consideration is not made in this respect, and this effect is not satisfactory. The number of thermally generated carriers depends on a defect level density, an interface level density, or the like, and essentially depends on a magnitude of a band gap. In general, however, it is known that the number of thermally generated carriers is decreased as the magnitude of the band gap is increased. However, this element is too narrow in minimum band gap to suppress the thermally generated carriers. A semiconductor light-receiving element having such a band gap is suitable for receiving light within a wavelength range of 1.0 .mu.m to 1.6 .mu.m, but is not suitable for light-receiving elements in other wavelength ranges, e.g., for a visible light light-receiving element, resulting in a limited application field.
In a combination of the second materials, although an ionization energy is as high as about 1 eV, a conduction band energy step at a stepback is as small as about 0.6 eV. Thus, the second materials are not promising ones.
Other materials described above also have the same drawbacks as in the first materials. In particular, in the last combination of materials, an article by T. P. Pearsall described in Electronic Letters Ver. 18, Vol. 12 (June, 1982), pp. 512-514 proposed an element having a minimum band gap of 0.5 eV and a maximum band gap of 1.3 eV by changing a composition ratio of Hg to Cd. However, this element has a very narrow minimum band gap, and is more easily influenced by a thermally generated dark current.
Therefore, in order to effectively put a low-noise APD having a structure for increasing a carrier ionization ratio into practical applications, the degree of freedom in selection of manufacturing methods, suppression of a dark current, a band-structure having a wide light-receiving wavelength range, and the like must be taken into consideration.
More specifically, when technical problems to be solved of the above-mentioned APD are summarized, the following technical problems on performance and manufacture are given.
The technical problems on performance of the element are as follows:
(1) Since incident light is absorbed by the p-type semiconductor layer and the multiplication layers, a multiplication ratio is changed depending on an incident wavelength of light, and is not suitable for a reading element. PA1 (2) Since light absorption layers and multiplication layers have small forbidden bandwidths, a dark current during an operation is high, and noise is large. PA1 (3) Since the element aims at optical communication, materials are limited, light to be coped with falls within a range of about 800 to 1,600 nm, and light in other wavelength ranges such as visible light cannot be coped with. PA1 (1) In order to form a stepback structure using compound semiconductors, it is difficult to modulate compositions, and magnitudes of .DELTA.Ec and .DELTA.Ev are limited, thus restricting a low-noise structure. PA1 (2) Since compound semiconductors belonging Groups III-V, II-IV, and the like are used as materials, there are problems as industrial materials such as toxicity and material costs. PA1 (3) A formation method of compound semiconductors suffers from various problems. For example, an ultra high vacuum must be set, film formation must be performed at high temperatures (about 500 to 650.degree. C.), a large-area structure is difficult to attain, etc. Therefore, this method is not suitable for a manufacturing method of a reading element. PA1 wherein the forbidden bandwidth Eg1 of the light absorption layer is set to be substantially equal to the maximum forbidden bandwidth Eg3 of the multiplication layer. PA1 wherein the forbidden bandwidth Eg1 of the light absorption layer is set to be substantially equal to the minimum forbidden bandwidth Eg2 of the multiplication layer. PA1 wherein the forbidden bandwidth Eg1 of the light absorption layer is continuously changed from a side of one electric charge injection blocking layer stacked on the light absorption layer so as to be substantially equal to the maximum forbidden bandwidth Eg3 of the multiplication layer on a side of the multiplication layer. PA1 wherein the forbidden bandwidth Eg1 of the light absorption layer is continuously changed from a side of one electric charge injection blocking layer stacked on the light absorption layer so as to be substantially equal to the minimum forbidden bandwidth Eg2 of the multiplication layer on a side of the multiplication layer. PA1 wherein the photoelectric conversion apparatus is constituted in such a manner that a light absorption layer, having a forbidden bandwidth Eg1, for absorbing light to generate photocarriers, and a multiplication layer, obtained by stacking one or a plurality of layers having a stepback structure in which a forbidden bandwidth is continuously changed to have a minimum forbidden bandwidth Eg2 and a maximum forbidden bandwidth Eg3, for multiplying the carriers generated upon light absorption, are stacked to be sandwiched between an electric charge injection blocking layer stacked on the light absorption layer and a substrate on which the signal output unit is formed, and which has an electric charge injection blocking function.
The technical problems upon manufacture of the element are as follows: